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Key card doesn't make any logical sense to me but I don't know that quantitative does either.

3♥ does not show six hearts. Hands with 4♣ + 6♥ don't raise clubs and a 6♥ 5♣ hand should just splinter or something.

If we had Steve's 3=2=4=4 with spades stopped, why aren't we making a quantitative call on the prior round. 3♥ can't signal any ‘awakening’ on responder's part. Now I suppose 3♦ could be a stab to see if opener can play the hand - I am thinking ♠Axx ♥Qx ♦AKxx ♣AQxx or so, but this feels like a reach.

Similarly, 3♦ and then 4N does not make much sense either. if Responder has a pure RKC hand, why futz around with 3♦?

Many average/average plus players stick to the hard and fast rule that 4N is RKC for our last bid suit, and I can see a club or NT slam drive that might as well ask about the ♥K along the way and doesn't care about clubs.